1. Field of the Invention
The present invention relates generally to radio-frequency (RF) and/or microwave components, and particularly to RF and/or microwave coupled transmission line components.
2. Technical Background
A coupler is a four-port passive device that is used to combine, split and/or direct an RF signal within an RF circuit in a predictable manner. A coupler may be implemented by disposing two transmission lines in relatively close proximity to each other. The main transmission line includes a first (input) port and a second (output) port. The secondary transmission line includes a third (output) port and a fourth (isolation) port. Couplers are designed to sample an RF signal, and output a coupled signal having predetermined signal characteristics (e.g., amplitude, frequency, phase, insertion loss, etc.).
Directional couplers operate in accordance with the principles of superposition and constructive/destructive interference of RF waves. When coupling occurs, the RF signal directed into the input port of coupler is split into two RF signals. A first portion of the RF signal is available at the second port and a second portion of the RF signal is available at the third port. The amount of RF signal power in the first and second output signals should equal the RF signal power of the input signal. However, the coupler usually has an “insertion loss” which accounts for the differences between the input signal and the output signals. The coupled output signal and the direct output signal are out of phase with respect to each other. At the isolation port, there is destructive interference of RF waves and the RF signals cancel such that there is no appreciable signal available at the fourth port. In practice, the cancellation is not perfect and a residual signal may be detected. The residual signal at the isolation port is another measure of the performance of the device. Hybrid couplers are commonly used in many wireless technologies to divide a power signal into two signals. In many instances the size of the coupler is critical for both application requirements and material cost benefits.
More formally, coupler structures can typically be described as two transmission lines of length/with an even and odd mode impedance, Z0E and Z0O. The length of the coupler may be put in terms of the dielectric constant (∈R) of the material used to implement the transmission line in accordance with the following formula:
  l  =      c          4      ⁢              f        0            ⁢                        ɛ          r                    Where c is the speed of light and f0 is the desired center frequency.The even mode impedance is the line impedance when the two coupled lines are at the same electric potential. The odd mode impedance is the line impedance when the lines have opposite electric potential. The overall system impedance of the coupled line pair is given by:Z0=√{square root over (Z0EZ0O)}The coupling factor, k, is given from the even and odd mode impedance parameters:
  k  =                    Z                  0          ⁢          e                    -              Z                  0          ⁢          o                                    Z                  0          ⁢          e                    +              Z                  0          ⁢          o                    
To achieve a tight coupling factor the even mode impedance must be relatively high and the odd mode impedance should be relatively low, while maintaining the proper system impedance. A 3 dB coupler in a 50 ohm system has an even mode impedance of approximately 120.7 ohms and an odd mode impedance of approximately 20.7 ohms. The length of the coupled lines is chosen to be a quarter wavelength (90°) long at the coupler's operating frequency (f0) (i.e., the frequency of the RF signal being divided or combined).
There are various methods commonly used to implement a coupler structure. For example, broad side coupled lines and edge-side coupled lines are simple structures used to create couplers. The basic edge-side or broadside coupler may be modified to vary coupling value using various methods including re-entrant structures, spirals, meandered lines and lumped elements among others.
The broadside coupled line pair is a simple distributed coupling structure which can easily achieve tight coupling. This design is simply two stripline layers arranged together on top of one another, without a ground plane layer between the coupled line pair. Meandered broadside coupled lines are similar to the broadside coupled lines, but differ in that the broadside coupled line pair is disposed in a meandered pattern in an effort to achieve a more compact hybrid coupler. A design somewhat similar to the meandered pattern is the simple spiral design. If the spiral turns are arranged closely together then tight coupling is particularly easily achieved due to the minimized even mode fringing fields of each individual turn, resulting in a relative higher even mode impedance.
Each of the broadside coupled lines pairs described above have their advantages as well as disadvantages. From an impedance standpoint, the straight broadside coupled line pair and the meandered coupled line pairs both have symmetrical ports. Edge-side and/or broadside coupler topologies have drawbacks in that they are relatively large in the x-y direction because of the line length required. These types of designs are also characterized by a relatively large dimension along the z-axis to achieve to correct even mode impedance. Meandered broadside coupled lines also have size and density limitations. When meandered lines are disposed side-by-side, the RF currents flowing in adjacent lines tend to oppose each other. Accordingly, adjacent lines cannot be arranged in a tight configuration. This reality, of course, limits the spatial density of meandered line configurations. What is needed is a coupler line configuration that provides an increased line density suitable for implementing miniaturized coupler devices.
A simple spiral configuration is a method that may be employed to achieve tight coupling in a small area. However, since there is no symmetry along the z-axis there is a varying impedance value for the inside and outside ports. This affects the coupler's ability to divide and combine RF signals, particularly in high efficiency amplifiers, where the amplifier impedance changes with the amplitude of the signal. A simple spiraled coupled line pair achieves tight coupling values and may be implemented in a miniaturized form factor. The spiral structure allows for tighter coupling and smaller lines and spaces, which allows the coupler to shrink in the x, y and z directions.
As alluded to above, the so-called spiral coupler structure has a serious drawback, however, since port locations are disposed on the inside and outside of the spiral. As those skilled in the art will understand, when the frequency increases, there is a wide divergence in the impedances of the various ports. This asymmetry seriously impacts the coupling performance of the device. What is needed, therefore, is a miniaturized coupler device that eliminates the impedance mismatch between coupler ports.
One approach that has been considered to remedy the asymmetric impedance issue described above is to employ several (e.g., at least four (4)) layers of spiraled transmission lines, coupled in groups and interconnected by transmission line vias that are disposed vertically between the at least four layers. RF signal traces are disposed on the top layer below the top ground plane, and between the vias and the device pins. However, an issue that may impact the performance of spiral couplers relates to the thermal resistance of the coupled structure. The heat generated by an RF signal as it traverses the signal traces must eventually be dissipated by the heat sink disposed underneath the device. The thermal resistance is proportional to the region disposed under the trace; the greater the vertical distance from the trace to the bottom of the device, the higher the thermal resistance. In this case, because the traces are disposed in the top layer, the thermal energy must be conducted through the entire thickness of the device. Alternatively ground planes may be disposed between the coupled groups and a thermal path realized through interconnecting vias towards the heat sink. The power handling capabilities of these devices is thus lower (i.e. a factor of approximately 0.4 to 0.9, depending on the actual thermal path) than having the entire spiral coupler disposed in one stripline layer. Accordingly, the RF signal power level must be lowered or the device will be impaired or destroyed by the heat generated by the RF signal itself.
To someone skilled in the art, it is obvious that the thermal path is only part of the equation. Equally important, if not more, is the insertion loss of the device. The insertion loss represents the amount of energy that will be dissipated in the device. Thus minimizing the insertion loss will result in less heat being generated in the device, and therefore, less heat needs to be conducted to a heatsink. A device will have a set maximum operating temperature that can be a function of the material characteristics such as the reflow temperature of the port terminal solder used, permanent breakdown of dielectrics or metals as a function of temperature and environment, physical alterations as a function of temperature and environment or temporary performance changes versus temperature. A lower thermal resistance, higher maximum operating temperature and lower insertion loss are therefore the design goals of a high power device.
What is needed is a coupler structure that exhibits the compactness of a spiral coupler for cost, while simultaneously provides symmetric port impedances and high power handling capabilities.